a rock do any damage (other than perhaps cosmetic but I really never bother to look very hard) to the condensers. And I can assure you rocks have hit the car, one can see the numerous scars on the bumper cover and the hood and even the windshield. (The 2nd one, the original finally developed a full width crack from a rock hit and had to be replaced.)
I suspect more often what happens is the thing develops a leak due to corrosion. Keep those radiator ducts free of trash/litter. Or the driver pulls too far in and runs the nose over a high curb and this damages the condenser or radiator. (Lost both on the driver's side to an encounter late at night with a Detroit Alligator (tire carcass).
IIRC Porsche forbids any screens at the radiator intakes though what Porsche forbids often is ignored more often than not.
You have to worry about how much surface area a screen/mesh takes up. There are charts (and formulas even) that give some numbers for area reduction based on the area of the mesh screen and its wire side and wire spacing. The reduction can be upwards of 30% to 40% or more.
Also, there is the issue about a piece of paper blowing up and covering the opening. I've seen a number of other make/model cars on the road with a newspaper or plastic bag blocking the radiator. In the case of the Boxster the opening is not that large so a full blockage of one side is possible. While I have driven both cars with one non-operating radiator fan with no overheating issues I'm not anxious to drive either car with one radiator duct fully blocked off to the point no air would flow through.
Last there is a false sense of security. The mesh may stop the rocks but it won't stop the dirt/sand and smaller particles of dirt. These will accumulate, pack and retain moisture and corrosion of the condenser and radiator will still occur, maybe at an accelerated rate as the larger fluffy stuff (blocked by the mesh) tends to keep the smaller denser stuff from becoming as compacted. Because the build up is not visible you will let the cleaning of these ducts go thinking all is ok until a leak develops.